Well, Keirsey makes another mistake. He lists George W. Bush as an Artisan. GWB is clearly ESFJ, and with Keirsey's own methods of recognizing hand and speech patterns this is obvious.
Then againg, Keirsey mistyped Jung as an NF, when he is obviously INTP (even said so himself, in his own words, though van der Hoop disagrees) and he also has no notion of the difference between Introvert and Extravert. Making them just shy/not shy, which is ludicrous. Keirsey considers himself INTP, though i'm relatively sure he's actually just a shy ENTP, which would explain the genius is mapping the four-types and the MBTI (ENTPs are great at making different things work together), the misunderstanding of introverts (only an extravert would think an introvert is "shy"), his horrid writing (INTP are technical and short in writing style, which is nothing like Keirsey), and he fantastic ideas about how the types use speech and hand motions (an extraverted trait, especially extraverted intuitive (ENTP/ENFP) which will easily recognize patterns in the outside world).
With all that, his book (Please Understand Me II) is quite good, and i'm planning on getting his earlier books (Please Underastand Me, Portraits of Temperament, and Presidential Temperaments) because he notices things very well, and most of what he says is on the mark.
I guess the worst part of his whole idea is that he doesn't recongize that types develop throught their life (Joland Jacobi explains it very well in The Psycology of C G Jung) and he basically addresses the immature or underdeveloped types. Anyone who has gone though mid-life (that is the "crises" and did not shy away from it) will see themselves as more than one of Kiersey's types because they have matured.
It's too bad though. To see someone so close to the truth and then fall away.